Alright, I see this asked every once and a while. I thought I'd make a tutorial about it.

If you don't have a image editing program (I will be using Photoshop/Fireworks) here is the Finished .fla.

Open Photoshop, Fireworks or any other program that has some sort of "Noise" Filter. The concept is the same, just read one and you'll understand.

Let's get started!

In Photoshop:
1. Set the foreground color to black and the background color to white, or vice-versa. It doesn't really matter.
2. Filter>Noise>Add Noise...
3. In the Window that comes up change the "Amount" to around 300 (My favorite, feel free to play around), Distribution should be set to Gaussian and Monochromatic should be checked.
4. Now, Select All (Ctrl/Command+A). Copy (Ctrl/Command+C).
5. Open Flash
6. Make a new document and a new blank keyframe. Paste (Ctrl/Command+V).
7. Repeat steps 1-6 at least 4 times. Remember, the more you have the better it will look but the larger your file will be.

In Fireworks:
1. Make a new layer, fill it with black.
2. Filters>Noise>Add Noise...
3. Set Amount around 300 (Just my favorite, so feel free to play around), and uncheck Color.
4. Now, Select All (Ctrl/Command+A). Copy (Ctrl/Command+C).
5. Open Flash
6. Make a new document and a new blank keyframe. Paste (Ctrl/Command+V).
7. Repeat steps 1-6 at least 4 times. Remember, the more you have the better it will look but the larger your file will be.

Note, If the Copy/Pasting Method is not working just save each different noise thing as a different file and then after you make a couple File>Import>Import to Library into Flash. Drag from the Library onto their separate frames.

Nice, been looking for a tutorial on how to make one without photoshop for a while:) Well explained too, It probably wont get into AS : Main, because theres no AS. XD.
But it should get into one of the helpful links at the bottom 8). I think, lol.